Hydrogen is the cleanest and most efficient fuel used in fuel cells. It is widely used in low temperature fuel cells like proton-exchange membrane (PEM) fuel cells, alkaline fuel cells, and phosphoric acid fuel cells, since its oxidation rate at the anode is high enough even at room temperature. However, producing pure hydrogen is not a trivial task. Hydrogen is normally produced through reforming hydrocarbon fuels such as methane, propane, and methanol. This not only makes the entire fuel cell system more complicated, it also dramatically increases its cost. Moreover, any carbon monoxide (CO) remaining in the reformed gas, even at ppm levels, will poison the electrodes of a PEM fuel cell and reduce its performance. In addition, transporting and storing hydrogen can be dangerous and difficult.
The problems associated with hydrogen have encouraged scientists to look for other fuels that can be directly oxidized without utilizing a reforming step. Methanol, the simplest alcohol containing only one carbon atom, is the most popular and widely used alternative. A direct oxidation fuel cell using methanol as the fuel is called a direct methanol fuel cell (DMFC). DMFCs have a long history. U.S. Pat. No. 3,113,049, issued to Worsham et al. on Dec. 3, 1961 for DIRECT PRODUCTION OF ELECTRICAL ENERGY FROM LIQUID FUELS, describes liquid feed, direct, methanol fuel cells.
Early DMFCs have used liquid electrolyte such as dilute sulfuric acid, for proton transportation. Sulfuric acid, however, can cause major problems when used as a liquid electrolyte, one of which is corrosion of the fuel cell materials. Sulfuric acid can also poison electrodes by the adsorption of sulfate anions, and cause leakage of electrolyte through the surrounding materials. For example, the electrolyte could gradually leak out through the pores of the air cathode, which also causes fuel loss and cathode poisoning. In order to alleviate such a leaking problem, a solid proton-exchange membrane was interposed between the anode and cathode. Nafion®, a perfluorinated polymer, made by E. I. DuPont, was used by Kudo et al. who were issued U.S. Pat. No. 4,262,063 on Apr. 14, 1981 for FUEL CELL USING ELECTROLYTE-SOLUBLE FUELS; and by Kawana et al. who were issued U.S. Pat. No. 4,390,603 on Jun. 28, 1983 for METHANOL FUEL CELL. U.S. Pat. No. 4,478,917, issued to Fujita et al. on Oct. 23, 1984 for FUEL CELL, uses sulfonated styrene-divinylbenzene co-polymers as the membrane.
Current fuel cell practice eschews the use of liquid electrolyte in a DMFC. U.S. Pat. No. 5,599,638, issued to Surampudi et al. on Feb. 4, 1997 for AQUEOUS LIQUID FEED ORGANIC FUEL CELL USING SOLID POLYMER ELECTROLYTE MEMBRANE, uses a proton exchange membrane such as Nafion membrane as the electrolyte. Nafion membranes have excellent chemical, mechanical, thermal, and electrochemical stability, and their ionic conductivity can reach as high as 0.1 S/cm. The kinetics of methanol oxidation and oxygen reduction at the electrode/Nafion membrane/electrode interfaces have been found to be more facile than those at the previously used electrode/sulfuric acid/electrode interfaces. This also has the advantage of reducing corrosion. The cell could also be operated at temperatures as high as 120° C., compared with sulfuric acid cells that tend to degrade at temperatures higher than 80° C. Also, the absence of conducting ions in the fuel/water solution substantially eliminated the parasitic shunt currents in a multi-cell stack. U.S. Pat. No. 6,248,460 B1 granted to Surampudi et al. on Jun. 19, 2001 (a continuation of U.S. Pat. No. 5,599,638), describes this type of cell.
In U.S. Pat. No. 5,904,740, issued to Davis on May 18, 1999 for FUEL FOR LIQUID FEED FUEL CELLS, a cell was described comprising a formic acid and methanol/water solution for the conduction of protons within the anode structure. Formic acid appeared to improve ionic conductivity and was clean burning. Furthermore, it did not poison the catalysts.
Unfortunately, methanol still presents a serious cross over problem through the Nafion and other similar types of proton-exchange membranes. This result is due to its physical diffusion, and electro-osmotic drag by protons. Such a crossover not only results in a large waste of fuel, but also greatly lowers the cathode performance.
Most of the methanol that crosses over will be electrochemically oxidized at the cathode. Such oxidation reactions not only lower the cathode potential, but also consume some oxygen. Should the reaction intermediate, such as carbon monoxide, become adsorbed to the catalyst surface, the cathode will be poisoned, further lowering the cell performance.
U.S. Pat. No. 5,672,438, issued to Banerjee et al. on Sep. 30, 1997 for MEMBRANE AND ELECTRODE ASSEMBLY EMPLOYING EXCLUSION MEMBRANE FOR DIRECT METHANOL FUEL CELL, describes a fuel cell having a thin layer of polymer. The polymer has a higher ratio of backbone carbon atoms to that of the cationic exchange side chain. This polymer can reduce the methanol crossover rate, albeit at the expense of increasing the membrane resistance. It was suggested that the polymer with higher carbon atom ratios be preferably oriented on the anode side.
In PCT International Patent No. WO 98/22989, issued to Prakash et al. on May 28, 1998, a fuel cell having a polymer membrane composed of polystyrene sulfonic acid (PSSA) and poly(vinylidene fluoride) (PVDF) is described. Such a PSSA-PVDF membrane exhibited lower methanol crossover, which translated into a higher fuel and fuel cell efficiency.
In PCT International Patent No. WO 01/93361 A2, granted to Pickup et al. on Dec. 6, 2001, a modified ion exchange membrane that possessed lower methanol crossover is described. The cell contained modified membranes comprising Nafion. The modification was accomplished in situ, by polymerization of monomers, such as aryls, heteroaryls, substituted aryls, substituted heteroaryls, or a combination thereof. The modified membrane was observed to exhibit reduced permeability to methanol, and was often observed without a significant increase in ionic resistance. Another barrier to the commercialization of DMFCs has been the sluggish methanol oxidation reaction. Moreover, some intermediates from methanol oxidation, such as carbon monoxide, can strongly adsorb onto the surface of catalysts. This can cause them to be seriously poisoned. Pt alloys like Pt/Ru have a much higher CO tolerance, so they are widely used as the anode catalyst.
Other short chain organic chemicals such as formic acid, formaldehyde, ethanol, 1-propanol, 1-butanol, dimethoxymethane, trimethoxymethane, and trioxane have been speculated as being useful fuels for direct oxidation fuel cells. U.S. Pat. No. 5,599,638, issued to Surampudi et al. on Feb. 4, 1997 for AQUEOUS LIQUID FEED ORGANIC FUEL CELL USING SOLID POLYMER ELECTROLYTE MEMBRANE describes experimental results in the use of dimethoxymethane, trimethoxymethane, and trioxane for direct oxidation fuel cells. It was claimed that dimethoxymethane, trimethoxymethane, and trioxane could be oxidized at lower potentials than methanol, and thus, could be better fuels than methanol. It was also claimed that only methanol was found to be the intermediate product from the oxidation of these fuels, and thus, it was not a concern because methanol would be ultimately oxidized to carbon dioxide and water. Using Nafion 117 as the membrane and oxygen as the oxidant at a pressure of 20 psig, cell voltages of 0.25 V, 0.50 V, and 0.33 V were achieved at a current density of 50 mA/cm2 when dimethoxymethane, trimethoxymethane, and trioxane were used at cell temperatures of 37° C., 65° C., and 60° C., respectively.
Recently, Qi et al. discovered that secondary alcohols such as 2-propanol could perform much better than methanol in dilute aqueous solutions, especially at current densities less than approximately 200 mA/cm2. This result is described in copending U.S. patent application Ser. No. 10/091,624, filed on Mar. 5, 2002 now U.S. Pat. No. 7,049,014; and in Electrochemical and Solid-State Letters, pp. A129–A130, June 2002. The better performance of this fuel appears to be the result of the faster kinetic reaction of 2-propanol, and the lower tendency for alcohol crossover.
All the direct oxidation fuel cells reported so far use dilute aqueous solutions as their fuels. The concentration of the fuel in the solution is normally less than 10% (wt.), and most frequently about 3% (wt.). The other 90–97% (wt.) is water. This not only results in a bulky fuel cell system, but also seriously limits its energy density. In addition, a large amount of water from the anode compartment transports through the membrane to the cathode side, and causes serious flooding of the cathode.
In order to decrease this flooding, a high airflow rate is normally utilized. Using a high airflow rate, however, not only consumes more parasitic power, but also makes it difficult to balance the water within the fuel cell system. In the event that the water that is taken away by exhaust air should not be effectively recovered, a larger water reservoir will be needed for the same amount of fuel.
The present invention provides a direct oxidation fuel cell using neat fuels. This eliminates the need to carry large amounts of water, thus reducing the bulkiness of the fuel cell system. A preferred fuel for such a fuel cell system is neat 2-propanol. The use of this neat fuel enhances the performance and efficiency of the cell. Higher energy densities are possible, and cathode flooding is eliminated.